Investigations Of Film Formation Characteristics And Mass Transfer Intensification In The Horizontal Twin-shaft Agitating Devolatilizers

The removal of small molecule volatiles such as unreacted monomers,residual solvents,and oligomers from polymers is a necessary stage of the polymerization process,which is polymer devolatilization.It is a separation process controlled by thermodynamics and mass transfer.Hence,the special separation equipments are used for devolatilization.The difficulty of the separation process lies in strengthening the mass transfer process in highly viscous polymers.With the improvement of polymer properties,monomer and solvent recovery,environmental protection and health requirements,the volatile content in polymer should be lower and lower.Therefore,it is of great significance to design and optimize the polymer devolatilizers in order to develop devolatilization process with high efficiency and low energy consumptionComputational fluid dynamics(CFD)and experimental measurement technology have been used to investigate the hydrodynamics,film formation mechanism,mixing process and mass transfer characteristics in the horizontal twin-shaft agitating devolatilizers.Achieved results are as followsFilm formation process and mechanism in the horizontal twin-shaft disk reactor have been investigated by CFD software Fluent.There is an overlapping zone between two adjacent disks,and two kinds of liquid films are generated in the reactor.Free film is not affected by the overlapping zone and the scraped film is generated by the overlapping zone.The free film,greatly affected by the liquid viscosity and rotating speed,possesses similar characteristics with that in the single-shaft reactor.The scraped film is much thinner and more uniform than the free film for highly viscous liquid Compared with the liquid viscosity and the rotating speed,the disk clearance plays a more significant role in determining the thickness and the shape of the scraped filmThe mechanism on a rotating wheel with open window distinguishes itself from that on a regular solid disk.Bounded film on the spoke and ring zone having only one gas-liquid interface and unbounded film in the open window with two gas-liquid interfaces coexist on the rotating wheel under certain operation conditions.The minimum rotating speed for unbounded film formation decreases when the liquid viscosity increases.The rate of film deformation has been used to study the surface renewal characteristics.The absorption of CO2 in the syrup at a constant rate has been adopted to determine the volumetric mass transfer coefficient(kLa)of the reactor Results show that the average rate of surface renewal and kLa of wheel with open window is higher than that of solid disk,although the intensification by the open window is limitedA novel horizontal twin-shaft has been designed.The agitator consists of disk with open window and kneading bar.Results show that there exists the asymmetry free surface when part of the kneader is filled with highly viscous fluid.There is unbounded film both within the kneading bar and open window.The gas and liquid mass transfer area and surface renewal characteristics increases with the number of kneading bars Hence,kLa tends to increase.What counts is that the mass transfer performance of twin-shaft kneader is much higher that of twin-shaft disk reactor due to the special film formation and surface renewal characteristicsThe CFD software Polyfow has been used to investigate the hydrodynamics in the novel horizontal twin-shaft kneader.Mixing process in the reactor has been studied by means of particle tracking technique.There exists hardly no flow dead zone.The primary flow pattern dominates.Meanwhile,there exists the secondary flow pattern in the overlapping zone.The kneading bars can push more material in the angular direction and intermesh periodically in the overlapping zone.Hence,the fluid element undergoes intense compression,shearing,and stretching and reorientation.Mixing process in the kneader can be greatly enhancedDirect devolatilization processes in a List-type horizontal twin-shaft kneader have been studied for styrene-based thermoplastic elastomer in cyclohexane.It involves complicated rheological dynamics,phase transformation and mass transfer during the devolatilization processes.The final form of product is up to the kinds of styrene-based thermoplastic elastomer.Under operating devolatilization conditions in the List-type kneader,the residual volitiles in styrene-based thermoplastic elastomer SBS and SIS is 0.10%and 0.22%,respectively.